Transparent thermoplastics have replaced glass in many applications, e.g. transportation glazing, taillight and headlight lenses, corrective optical lenses, architectural glazing and the like. Transparent thermoplastic polymers are lighter and more shatter resistant than glass. Consequently, the use of transparent thermoplastics in transportation glazing leads to a reduction in vehicular weight which leads to an improved fuel economy. Further, the improved shatter resistance of the transparent thermoplastic imparts additional improved safety features when accidents occur that would normally shatter glass glazing or lenses.
However, transparent thermoplastics are not as hard as glass and consequently these materials have a tendency to scratch and mar with even ordinary use due to dust, contact with abrasives, cleaning equipment and weathering. Continuous scratching or marring results in reduced visibility or transparency of the material defeating its initial purpose. This leads to the need to replace the material. In fact, one widely used transparent thermoplastic, polycarbonate, is so soft in an untreated or uncoated state that soft paper tissues can scratch the surface. Since this material has received widespread application as a replacement material for eyeglass lenses, this can be a significant drawback.
There thus has developed a significant body of technology dealing with means of coating transparent thermoplastics, thereby producing transparent laminates, to improve their performance characteristics to be more similar to that of glass while at the same time retaining the weight advantages that the less dense thermoplastics intrinsically possess. U.S. Pat. No. 4,348,462 discloses a radiation curable hardcoat composition for such transparent thermoplastics that comprises colloidal silica, an acryloxysilane, and a non-silyl acrylate. This composition, as a coating, imparts improved abrasion resistance to transparent thermoplastics relative to the same thermoplastics that are not so coated, thus the term hardcoat. These generic formulations have been continuously improved but even recent U.S. patents still recite colloidal silica, an alkoxy silylacrylate (cf. acryloxysilane), and a reactive acrylic monomer (U.S. Pat. No. 5,468,789). Alternative formulations recite colloidal silica and a multi-functional acrylate (U.S. Pat. No. 5,075,348), which is primarily a diacrylate species.
While these compositions impart a certain hardness to the coating by virtue of the presence of the colloidal silica, which on a micro-scale could be assumed to mimic glass, the matrix binding the colloidal silica within the coating and also binding the coating to the transparent thermoplastic substrate is an organic polymer having a tendency to be hydrophobic.
As a hydrophobic coating, colloidal silica dispersed in a polymerized organic matrix such as an acrylate does not function well in moist environments. When exposed to water, either as rain, fog, mist or a humid atmosphere, condensation occurs on the surface of the transparent thermoplastic and the water tends to bead up. The phase boundary of the water beads on the surface of the transparent thermoplastic serves to reflect and refract light, which is undesirable. Further, water is transparent and has a different index of refraction from the transparent thermoplastic substrate, the beads of water on the surface display a lensing effect interfering with visibility. Thus, a hydrophilic coating having anti-fog, that is anti-beading properties for water, would have better optical properties when challenged by water. This is because as the hydrophilicity of the surface coating increased, condensed water on the surface would have an increasing tendency to form a sheet instead of a multiplicity of water droplets that interfered with visibility or transmission of light.
While anti-fog coatings are known, and have been used with some transparent thermoplastics, they have not been particularly abrasion resistant (U.S. Pat. No. 3,933,407). The anti-fog coatings disclosed in U.S. Pat. No. 3,933,407 were applied to flexible polyethylene films as well as to rigid articles manufactured from glass and transparent thermoplastics. However, these coatings were not particularly abrasion resistant, i.e. they contained no colloidal silica.